This series is split out from the earlier, larger series "mm: Generalize HVO for HugeTLB and device DAX" [1]. While the parent series generalizes vmemmap optimization across HugeTLB and device DAX, this subset addresses a single, self-contained step: switching device DAX to the section-based sparse-vmemmap optimization infrastructure introduced for HugeTLB.
After the HugeTLB conversion, optimized vmemmap state is described by the memory section and the sparse-vmemmap population path can allocate or reuse shared tail vmemmap pages based on that metadata. Device DAX still uses the older DAX-specific population model, including a separate tail vmemmap page reservation and architecture-specific logic to locate or populate reusable tail pages. This series makes device DAX use the same section-based model. Device DAX sets the section order from pgmap->vmemmap_shift before vmemmap population, uses the common per-zone shared tail vmemmap page, and drops the extra reserved tail page. The powerpc radix path is updated to use the same shared tail-page helper, so the generic and powerpc DAX paths follow the same reservation model. The first patches prepare the shared infrastructure by introducing a generic CONFIG_SPARSEMEM_VMEMMAP_OPTIMIZATION symbol, factoring out shared tail-page allocation, and keeping the special shared-tail struct page initialization local to sparse-vmemmap. The middle patches move device DAX onto that infrastructure by recording the device DAX compound page order in the memory section, using that section metadata to back generic device DAX mappings with the common per-zone shared tail page, exposing the shared helpers so the powerpc radix path can use the same model, and then dropping the extra DAX-only tail page reservation, and the now-unused section accounting arguments can be removed. The final patch updates the documentation for the new DAX layout. This is intended to be the third smaller step toward the broader HVO generalization. The wider HVO consolidation between HugeTLB and device DAX is left for follow-up series. [1] https://lore.kernel.org/linux-mm/[email protected]/ Muchun Song (11): mm/sparse-vmemmap: introduce CONFIG_SPARSEMEM_VMEMMAP_OPTIMIZATION mm/sparse-vmemmap: factor out shared vmemmap tail page allocation mm/sparse-vmemmap: open-code init_compound_tail() mm/sparse-vmemmap: prepare DAX vmemmap population for section orders mm/sparse-vmemmap: set section order for device DAX mm/sparse-vmemmap: switch device DAX to shared tail vmemmap pages mm/sparse-vmemmap: move HVO helpers to a public header powerpc/mm: switch device DAX to shared tail vmemmap pages mm/sparse-vmemmap: drop the extra tail page from device DAX reservation mm/sparse-vmemmap: drop unused section_nr_vmemmap_pages() arguments Documentation/mm: update DAX vmemmap deduplication docs Documentation/arch/powerpc/vmemmap_dedup.rst | 90 ++------- Documentation/mm/vmemmap_dedup.rst | 32 +-- MAINTAINERS | 1 + arch/powerpc/mm/book3s64/radix_pgtable.c | 124 +----------- arch/x86/entry/vdso/vdso32/fake_32bit_build.h | 2 +- drivers/dax/Kconfig | 1 + fs/Kconfig | 1 + include/linux/mm.h | 6 +- include/linux/mmzone.h | 23 ++- include/linux/page-flags.h | 5 +- include/linux/vmemmap-optimization.h | 88 +++++++++ mm/Kconfig | 3 + mm/hugetlb.c | 2 +- mm/hugetlb_vmemmap.c | 30 +-- mm/internal.h | 9 - mm/memory_hotplug.c | 6 +- mm/mm_init.c | 15 +- mm/sparse-vmemmap.c | 184 ++++++++---------- mm/sparse.c | 3 +- mm/sparse.h | 78 +------- 20 files changed, 244 insertions(+), 459 deletions(-) create mode 100644 include/linux/vmemmap-optimization.h base-commit: da6c37ed8beb273e3308e42d4bca3ce11b4432fa -- 2.54.0
